1. Field of the Invention
The present invention is directed to an input circuit for a field device for communication with bidirectional signals that are superimposed on an analog control signal.
2. Description of the Prior Art
Electrical field devices that employ only the standardized electrical unit signals--particularly a 4 through 20 mA current signal--have been known for a long time. More recent field devices offer expanded parameterization and monitoring possibilities including diagnostic analysis or comparison by simultaneous communication with the field device. Field devices that employ the 4 through 20 mA current signal in a conventional way, for example in the form of a two-conductor device, and overlay this current signal with a high-frequency signal for communication have become widespread.
Particularly devices according to the HART field communication protocol (HART=Highway Addressable Remote Transducer) have become established as quasi standard in this field. Such field devices can be divided into two categories, namely those that impress a current on the two-conductor loop as a transmission signal such as is the case, for example, given measuring transducers for temperature or pressure, and those that receive a current as a control signal such as, for example, given I/P converters or position regulators (known as positioners) for generating a physical quantity, for example a pressure or a valve position.
Particularly for those field devices that receive a current as control signal, the limitation of the energy available for field devices that are designed as two-conductor devices requires special input circuits, since the ohmic resistance must be relatively low in order to cause the consumed energy to be acceptably low. No simple input circuits are known for field devices that receive a current as a corrective signal and enable a superposed digital communication.